1. Technical Field
This invention relates to the technology of making hard tipped fluted drill bits, and more particularly to the art of high speed drilling in rigorous material such as cast iron.
2. Discussion of the Prior Art
Tool steel is generally used in the construction of drill bits for drilling cast iron workpieces at drill speeds up to 400 rpm. However, with increases in drill speed (i.e., 10,000-40,000 rpm at penetration rates of up to 40-300 inches/minute) the cutting tips of such tool steel wear excessively and limit the bit life to an uneconomical circumstance.
Tips of such bits have been fitted with inserts of a more wear resistant material such as cemented carbide In Japanese patent publication 116 795 (1979), a plurality of elements were used to make up a carbide tip, leading to balance and chattering problems.
In search of even harder edge materials with even greater wear resistance at higher drilling speeds, the prior art has turned to boron nitride which additionally has excellent thermal stability. As revealed in U.S. Pat. No. 4,679,971, sometimes the boron nitride can be planted as short blades on a backing strip inserted into the tip, the blades extending partly radially from the cutting zone into the centering zone of the drill bit. As shown in FIG. 3 of such patent, these blades mutually abut at the center axis of such tool. Juncture of the boron nitride and the steel bit resides in the centering zone which leads to considerable breakage and chipping. This same patent attempts to overcome such breakage by fitting a cap of boron nitride around the entire tip of the tool. This is undesirable from a fabrication and exorbitant cost standpoint. Moreover, such cap still lacks support for resisting transverse forces.
In U.S. Pat. No. 4,728,231, such support problem was recognized and suggested that the entire diametrically extending implant be made of cubic boron nitride. Unfortunately, cubic boron nitride is brittle and cannot be easily secured to other metals, such as by brazing. Cubic boron nitride is unbrazable because it does not melt at brazing temperatures.
What is needed is an improved method of making and improved drill bit construction that permits cubic boron nitride to be used as an edge material for such drill bit without suffering from chip and breakage problems as well as securement problems so prevalent with the prior art. Overcoming these problems will result in highly improved drilling quality at such high speeds accompanied by highly increased wear life.
SUMMARY OF THE INVENTION
The problems associated with the prior art have been overcome by constructing a diametrical blade implant of a two-phase material: a main phase of cemented carbide and insert phases of cubic boron nitride placed at high stress corners of such blade, the inserts being diffusion bonded to the carbide and being limited strictly to the radial cutting zone of the bit without penetrating the centering zone.
In particular, the method aspect of this invention comprises adapting a fluted drill bit for more effective drilling of cast iron workpieces, comprising the steps of: (a) providing, across the pointed end of the fluted drill bit, a diametrically extending slot; (b) forming a blade effective to snugly fit within the contours of said slot, said blade being comprised of cemented carbide powder within which is molecularly bonded a pair of inserts constituted of cubic boron nitride, said inserts being located at opposite outside corners of said blade and extending radially inwardly no greater than the cutting zone of said pointed end of the fluted drill bit; and (c) after seating said blade within said slot, fixedly securing the carbide surfaces of said blade to the walls of said slot. The interface between the steel of said bit and the cubic boron nitride of said blade is desirably limited to less than 2% of the total secured (brazed) area of the blade.
Preferably, the drill bit is comprised of high speed tool steel, such as ASTM M-2, and the blade is a dual phase material having a primary matrix phase of cemented carbide, such as ASTM C-2, and an insert phase of cubic boron nitride powder, the matrix and insert phases being commonly hot pressed at a temperature of 2500.degree. C. and a pressure of about one million psi to achieve diffusion bonding.
Advantageously, the pointed end of the tool bit has a conical end face with frontal cutting edges and lateral cutting edges along the tool's shank separated by flutes forming helical cutting edges, the frontal cutting edges and lateral cutting edges intersecting at opposite sides of said tool bit to form corners at which the cubic boron nitride inserts are located.
Preferably, the means by which the cemented carbide is secured to the tool steel is by way of brazing with a composition consisting primarily of silver.